Valentines Day is often linked with that cheesy holiday for lovers, but that day can also be used to show your friends and family how much you appreciate them. So if you're going to give your sister a Valentine card, write why you love her, what makes her special to you, and what makes her such a great sister. Think about the good things about her. What you write should tell her how much you value her positive traits. In other words, this should be a meaningful sign of appreciation but doesn't cross the line of creepy (unless you two are close and are fine joking about that). Good luck!
Valentines Day is usually a day reserved for sending cards and gifts to lovers, I have never heard of anyone sending a Valentines Card to one's own sister..